Epidural anesthesia is an anesthesia method for injecting a local anesthetic into the epidural space to perform a reversible blocking in the spinal nerve. Nowadays, the epidural anesthesia is implemented in clinics by applying the “loss-of-resistance” method by injecting saline or air through a syringe connected to the epidural needle to confirm whether the epidural needle entering into the epidural space. Anesthesiologists rely on the feeling of the thumb pressure during the pushing of the loss-of-resistance syringe to determine the location of the epidural needle. When the epidural needle pierces through the ligamentum flavum and enters into the epidural space, air or saline in the needle would be easily injected into the epidural space under moderate pressure applied by the anesthesiologist because the resistance is disappeared. Thus, it can be confirmed whether the epidural needle is located in the epidural space. However, the mentioned method is not objective and must rely on a long-term anesthesia experience.
Recently, the positioning method for the epidural space by using a combination of the force impedance and the electrical impedance has been developed, such as a Taiwan Patent No. M338050. There are differences in the force impedances and electric impedance when the epidural needle is inserted into different subcutaneous tissues of human. By using the mentioned character of the force and electrical impedance differences, the location of the epidural needle could be determined if it is inserted into the epidural space. However, the distance between the epidural needle and the epidural space can not be reported by the above method.
U.S. Pat. Nos. 4,887,606 and 5,259,385 disclose a needle for positioning the blood vessel, and these patents apply a Doppler ultrasound to position the blood vessel. However, the Doppler ultrasound positioning method is not suitable for positioning the epidural space.
Although most current methods can indeed confirm the location of the epidural needle in the epidural space, by measuring the pressure change or electric impedance change. However, those methods can not tell the distance between the epidural needle and the epidural space before the needle reaches the epidural space. Thus, it is needed for a technique which can measure the distance in real time and therefore can alert the anesthesiologist to carefully advance the needle when it nears the epidural space, and avoid the accidental dural puncture, which causes severe headache in most patients.